Is it enough for a flood routing in a one reach river,to enter an initial flow data[the flow data before the flood routing], in the first station of the reach, as a initial conditions?
Or it also requires to enter a starting elevation for any storage areas?
If you have a storage area you must enter an initial starting elevation. It could be as simple as the invert elevation (minimum elevation) of your storage area to replicate a completely dry stroage area, if you want. If you only have one river and reach, an initial flow value at the upstream end of your reach is all you need.